The present invention relates to an optical information recording medium for recording and reproducing information with a large capacity using a laser beam and a method for initializing the same.
Examples of an optical information recording medium capable of recording and reproducing signals using a laser beam include a phase change type optical disk, a magneto-optical disk, and an optical disk using a dye material. In a recordable/erasable phase change type optical disk, a chalcogenide typically is used as a recording layer material. Generally, the crystalline state of the recording layer material is used as a non-recorded state. Signals are recorded by irradiating the recording layer material with a laser beam so as to melt and quench the recording layer material so that the material becomes amorphous. On the other hand, in order to erase signals, the recording layer is irradiated with a laser beam at a power lower than that for recording so that the recording layer becomes crystalline. Further, in order to reproduce recorded signals, the recording layer is irradiated with a laser beam at a power that does not cause the phase change of the material.
The recording layer made of a chalcogenide is formed in the amorphous state. Therefore, it is necessary to entirely crystallize a recording region beforehand to obtain a non-recorded state. Such entire crystallization of the recording region is called initialization. In general, the initialization process is incorporated in the disk production process and carried out using a laser beam source or a flash light source.
A recording medium with a so-called one-face-double layer structure has been proposed for the purpose of increasing the recording capacity of a recordable or a recordable/erasable phase change type optical disk. An optical disk with this one-face-double layer structure includes two information layers (multilayered films including recording layers) formed on a substrate. Signals are recorded or erased by a laser beam radiated through the substrate (i.e., radiated from the same direction).
In the phase change type optical disk with the one-face-double layer structure, when a first information layer on the side near the laser beam incident surface (i.e., the side near the substrate) is designed to have a xe2x80x9cHi to Lo structurexe2x80x9d (H-L structure) in which a reflectance of the layer is decreased by recording (formation of an amorphous mark), and a second information layer distant from the laser beam incident surface (i.e., distant from the substrate) is designed to have a xe2x80x9cLo to Hi structurexe2x80x9d (L-H structure) in which a reflectance of the layer is increased by recording (formation of an amorphous mark), the disk can serve as an optical information recording medium in which recording sensitivities of the first and second information layers and levels of the signals reproduced from these layers are uniform.
In the initialization process for the phase change type optical information recording medium with the one-face-double layer structure, what is important is whether stable focus control of the laser beam for initializing respective information layers is possible or not. However, in the medium including the first information layer with the H-L structure and the second a information layer with the L-H structure as described above, the second information layer presents a higher reflectance when the recording layer contained therein is amorphous than when it is crystalline, while the first information layer presents a lower reflectance when the recording layer contained therein is amorphous than when it is crystalline. Quantities of laser beams reflected from the first and second information layers are thus ill-balanced. Hence, when the recording layers in the recording region are entirely crystallized, it is difficult to obtain focus error signals from the first information layer while they are easily obtained from the second information layer.
With regard to initialization using a flash light source, the method conducted for a medium including a single information layer is already known (JP 62(1987)-250533 A). When the flash light source is used, the medium can be initialized in a shorter time period as compared with the case where a laser beam source is used.
However, in the optical information recording medium with a one-face-double layer structure, an attempt has never been made to initialize two information layers by a flash light radiated from the same direction. Unlike a laser beam, a focus of the flash light cannot be controlled. Furthermore, when the flash light is radiated from the substrate side for example, the second information layer has to be initialized by the light transmitted through the first information layer. In addition, different conditions should be adapted for initializing the first and second information layers because these layers generally have different structures in consideration of the fact that information signals are recorded/reproduced thereon/therefrom by the laser beam radiated from the same direction.
Therefore, in general, it is considered that the initialization using a laser beam is more appropriate for initializing the optical information recording medium with a one-face-double layer structure. However, initialization of the optical information recording medium with a one-face-double layer structure using the laser beam radiated from the same direction brings about some problems that do not occur in the case of the medium with a single layer structure.
An object of the present invention is to provide an initialization method suitable for an optical information recording medium with a one-face-double layer structure. It is another object of the present invention to provide a new optical information recording medium suitable for this initialization method.
In order to achieve the above-mentioned objects, the first optical information recording medium according to the present invention includes a substrate and at least a first information layer and a second information layer, and the first information layer and the second information layer, on/from which signals can be recorded/reproduced, are formed on the substrate in this order. The first optical information recording medium is characterized in that an area where the first information layer exists and the second information layer does not exist is provided on the substrate.
In the above-mentioned optical information recording medium, it is preferable that the substrate has a disk shape and the area where the first information layer exists and the second information layer does not exist is provided at least at one portion selected from an outermost circumference portion and an innermost circumference portion of an area where the first information layer exists. Furthermore, it is preferable that the first information layer includes a first recording layer and the second information layer includes a second recording layer, and the first and second recording layers are formed of a material that changes reversibly between an amorphous state and a crystalline state by irradiation of a laser beam from a side of the substrate.
In the above-mentioned optical information recording medium, with respect to a laser beam having a wavelength of xcex0 radiated from the side of the substrate for reproducing signals recorded on the first and second recording layers, a reflectance R0 (1amo) of the first information layer when the first recording layer is in an amorphous state, a reflectance R0(1cry) of the first information layer when the first recording layer is in a crystalline state, a reflectance R0(2amo) of the second information layer when the second recording layer is in an amorphous state, and a reflectance R0(2cry) of the second information layer when the second recording layer is in a crystalline state preferably satisfy the relationships:
R0(1amo) less than R0(1cry), and R0(2amo) greater than R0(2cry).
Further, in order to achieve the above-mentioned objects, the first method for initializing an optical information recording medium according to the present invention initializes an optical information recording medium by radiating a laser beam, the recording medium including a substrate and at least a first information layer and a second information layer, the first information layer and the second information layer, on/from which signals can be recorded/reproduced, being formed on the substrate in this order, wherein an area where the first information layer exists and the second information layer does not exist is provided on the substrate, the laser beam being irradiated from a side of the substrate. The first initialization method is characterized in that initialization of the first information layer is started from the area.
In the above-mentioned first initialization method, with respect to a laser beam having a wavelength of xcex1 for initializing the first information layer, a reflectance R1(1before) of the first information layer before being initialized, a reflectance R1(1after) of the first information layer after being initialized, a reflectance R1(2before) of the second information layer before being initialized, a reflectance R1(2after) of the second information layer after being initialized, and a transmittance T1(1before) of the first information layer before being initialized preferably satisfy the relationships:
R1(1before) less than R1(1after), R1(1before) less than R1(2before)xc3x97[T1(1before)]2, and R1(1before) less than R1(2after)xc3x97[T1(1before)]2.
Furthermore, in order to achieve the above-mentioned objects, the second method for initializing an optical information recording medium according to the present invention initializes an optical information recording medium by radiating a laser beam, the recording medium including a substrate and at least a first information layer and a second information layer, the first information layer and the second information layer, on/from which signals can be recorded/reproduced, being formed on the substrate in this order, the laser beam being radiated from a side of the substrate. The second initialization method is characterized in that, at least in an area where both the first and second information layers are formed, the first information layer is initialized after the second information layer is initialized.
In the above-mentioned second initialization method, with respect to a laser beam having a wavelength of xcex2 for initializing the second information layer, a transmittance T2(1before) of the first information layer before being initialized and a transmittance T2(1after) of the first information layer after being initialized preferably satisfy the relationship:
T2(1before) greater than T2(1after).
In the above-mentioned second initialization method, with respect to a laser beam having a wavelength of xcex1 for initializing the first information layer, a reflectance R1(1before) of the first information layer before being initialized, a reflectance R1(2before) of the second information layer before being initialized, a reflectance R1(2after) of the second information layer after being initialized, and a transmittance T1(1before) of the first information layer before being initialized; and, with respect to a laser beam having a wavelength of xcex2 for initializing the second information layer, a reflectance R2(1before) of the first information layer before being initialized, a reflectance R2(2before) of the second information layer before being initialized, and a transmittance T2(1before) of the first information layer before being initialized, preferably satisfy the relationships:
R1(1before) less than R1(2before)xc3x97[T1(1before)]2, R1(2before) greater than R1(2after), and R2(1before) less than R2(2before)xc3x97[T2(1before)]2.
In the above-mentioned second initialization method, it is preferable that the wavelength xcex1 of the laser beam for initializing the first information layer is equal to the wavelength xcex2 of the laser beam for initializing the second information layer.
Still further, in order to achieve the above-mentioned objects, the third method for initializing an optical information recording medium according to the present invention initializes an optical information recording medium by radiating a laser beam, the recording medium including a substrate and at least a first information layer and a second information layer, the first information layer and the second information layer, on/from which signals can be recorded/reproduced, being formed on the substrate in this order, wherein an area where the first information layer exists and the second information layer does not exist is provided on the substrate, the laser beam being radiated from a side of the substrate. The third initialization method is characterized in that initialization of the first information layer is started from the area, and the first information layer is initialized after the second information layer is initialized at least in an area where both the first and second information layer are formed.
In the above-mentioned third initialization method, with respect to a laser beam having a wavelength of xcex1 for initializing the first information layer, a reflectance R1(1before) of the first information layer before being initialized, a reflectance R1(1after) of the first information layer after being initialized, a reflectance R1(2before) of the second information layer before being initialized, a reflectance R1(2after) of the second information layer after being initialized, and a transmittance T1(1before) of the first information layer before being initialized; and, with respect to a laser beam having a wavelength of xcex2 for initializing the second information layer, a transmittance T2(1before) of the first information layer before being initialized and a transmittance T2(1after) of the first information layer after being initialized, preferably satisfy the relationships:
R1(1before) less than R1(1after), R1(1before) less than R1(2before)xc3x97[T1(1before)]2, R1(1before) less than R1(2after)xc3x97[T1(1before)]2, and T2(1before) greater than T2(1after).
In the above-mentioned third initialization method, with respect to a laser beam having a wavelength of xcex1 for initializing the first information layer, a reflectance R1(1before) of the first information layer before being initialized, a reflectance R1(1after) of the first information layer after being initialized, a reflectance R1(2before) of the second information layer before being initialized, a reflectance R1(2after) of the second information layer after being initialized, and a transmittance T1(1before) of the first information layer before being initialized; and, with respect to a laser beam having a wavelength of xcex2 for initializing the second information layer, a reflectance R2(1before) of the first information layer before being initialized, a reflectance R2(2before) of the second information layer before being initialized, and a transmittance T2(1before) of the first information layer before being initialized, preferably satisfy the relationships:
R1(1before) less than R1(1after), R1(1before) less than R(2before)xc3x97[T1(1before)]2, R1(1before) less than R1(2after)xc3x97[T1(1before)]2, R1(2before) greater than R1(2after), and R2(1before) less than R2(2before)xc3x97[T2(1before)]2.
Also in the above-mentioned third initialization method, it is preferable that the wavelength xcex1 of the laser beam for initializing the first information layer is equal to the wavelength xcex2 of the laser beam for initializing the second information layer.
The present invention also provides an initialization method using a flash light. That is, the fourth method for initializing an optical information recording medium according to the present invention initializes an optical information recording medium, the recording medium including a substrate and at least a first information layer and a second information layer, the first information layer and the second information layer, on/from which signals can be recorded/reproduced, being formed on the substrate in this order, and the method is characterized in that the first and second information layers are initialized by at least one flash light radiated from a light source provided on the side of the substrate.
In the above-mentioned fourth initialization method, it is preferable that the first and second information layers are initialized at the same time by a flash light with an emission time of not more than 200 xcexcs (xcexcsecond) radiated once.
In this case, it is preferable that the first and second information layers are initialized at the same time by a flash light radiated once from a light source with emission energy of not less than 1.5 J/cm2 and not more than 1.8 J/Cm2 arranged at a distance of not less than 5 mm and less than 15mm from the optical information recording medium. Additionally, it is preferable that the flash light passes through the first information layer at a transmittance of not less than 40% and not more than 70% .
In the above-mentioned fourth initialization method, a plurality of flash lights may be radiated.
In this case, it is preferable that at least two flash lights that are different in emission energy are radiated. Further, it is preferable that the second information layer is initialized by a first flash light, and then the first information layer is initialized by a second flash light with emission energy lower than that of the first flash light. The above-mentioned method including a plurality of times of flash light irradiation is suitable for a disk-shaped optical information recording medium having a radius of not less than 50 mm.